Secondary cells such as nickel-cadmium batteries may be recharged many times throughout their useful life. The recharging operation must be carefully controlled to minimize detrimental effects on the battery which are well known to those skilled in the art (see for example "Battery Charging: Extending Life Capacity", Bob Williams, Cellular Business, April, 1989, pp. 44-49). In the early days of secondary cell recharging technology, the recharging operation took several hours. With the increasing popularity of consumer devices powered by secondary cells, a demand has arisen for systems capable of recharging secondary cells in minutes instead of hours. Although it is possible to "fast charge" a secondary cell, this requires even more careful monitoring and control of the battery recharging process in order to prevent irreversible damage to the battery (see for example "Nickel-Cadmium Battery Update 90", Report on September, 1990 Brussels Seminar by Cadmium Association, London, England, November, 1990).
The prior art has evolved a wide variety of secondary cell recharging systems capable of rapidly recharging secondary cells. These typically involve electrical circuits which monitor the voltage and/or temperature of the battery being recharged and discontinue and/or vary the application of charging current to the battery once its temperature or voltage reaches a predetermined level. U.S. Pat. No. 4,006,397 Catotti et. al. is typical of the prior art.
Japanese Examined Patent Publication (Kokoku) Nos. 62-23528 and 62-23529 disclose methods of recharging secondary batteries such as nickel-cadmium batteries, wherein attention is given to a change in the voltage waveform of a cell during the recharging operation, a plurality of inflection points appearing in the voltage waveform are stored in advance, and the charging operation is discontinued when the stored plurality of inflection points develop according to a predetermined order. According to the above methods, however, it is necessary to record in advance a change in the voltage waveform during the charging operation for each of the batteries of various kinds and to rewrite the stored content to the one that corresponds to the battery that is to be recharged depending upon the kinds of batteries that need recharging, involving a cumbersome operation. Depending upon the environment in which the charging operation is carried out and the hystevesis of the battery, furthermore, the waveform of voltage output of the battery does not necessarily exhibit the order or the magnitude that are stored making it difficult to correctly carry out the charging operation or the recharging operation. It is therefore difficult to execute a rapid charging operation without deteriorating performance of the batteries.
That is, secondary batteries and, particularly, nickel-cadmium batteries have heretofore been recharged usually requiring a time of from 6 hours to 16 hours in the longest case. Even in the case of a so-called rapid recharging in which the recharging is carried out within a relatively short period of time, a time of one to two hours is required.
When the so-called rechargeable cells, storage batteries and cells are used for their respective purposes by being recharged, it is desired that they are recharged in as short a time as possible. However, a bottleneck exists in that the temperature rises and the internal pressure rises due to a chemical reaction inside the secondary batteries. Recharging by flowing a heavy current within a short period of time results not only in damage to the cells but also in deterioration of cell characteristics such as output characteristics and charging characteristics, and thus has not been employed.
In recent years, however, a damand for secondary batteries is increasing in a variety of industrial fields. Depletion of a power source during operation must be avoided as much as possible, and recharging of secondary cells rapidly or, more desirably, instantaneously, has been desired more than ever before particularly where machine tools are used, in hospitals where medical equipment is used and in communications businesses inclusive of portable telephones.
The object of the present invention therefore is to improve on defects inherent in the above-mentioned prior art and to facilitate the recharging of secondary batteries and, particularly, nickel-cadmium batteries within such short periods of time as about several minutes to less than 20 minutes. Recharging at this very high rate increases the significance of certain parameters which are not as significant in slower, prior art recharging systems. However, it has been found that these parameters can be effectively managed to yield a safe, rapid recharging system without subjecting the battery to detrimental side effects.